Treatment of certain medical conditions and diseases such as hydrocephalus frequently includes implanting a subcutaneous ventriculoperitoneal (VP) shunt to drain cerebrospinal fluid (CSF) from a ventricle of the brain to a receiving cavity, such as the heart or peritoneum. A VP shunt typically includes a catheter which extends from the ventricle, through a burr hole in the skull located behind the ear, and along the side of the body to the abdomen.
Early VP shunts exhibited a performance characteristic known as "siphoning" which refers to surging of CSF drainage when the patient changes from a recumbent to a sitting or standing position. Siphoning is caused by the rapid increase in hydrostatic pressure differential in the catheter due to the sudden change in height of the head above the abdomen. In addition to temporary discomfort, excessive CSF drainage might cause other adverse consequences, for example, distention of the brain, or rupture of the blood vessels leading to potentially serious brain hematoma.
To eliminate siphoning, VP shunts sometimes incorporate an antisiphoning valve which continuously senses the patient's attitude (i.e., horizontal-to-vertical orientation) and automatically moderates flow when the patient moves to an upright position. A weighted ball check valve is the operative mechanism frequently used in conventional antisiphoning valves. Generally in such valves, one or more spherical balls which are more dense than CSF reside within a hollow cavity of an elongated housing with a seat at the CSF inlet end. The antisiphoning valve is implanted so that the housing is horizontally oriented when the patient lies down. In the horizontal orientation, the balls can roll away from the seat, thus allowing CSF to pass through the valve. When the patient sits or stands, the housing orients vertically with the seat at the bottom. The balls sink toward the seat, thereby stopping flow until the aggregate intracranial and hydrostatic pressures of fluid in the catheter exceeds the force exerted by the weight of the balls against the seat.
A patient also is susceptible to experience CSF surging while recumbent. When a patient coughs or sneezes, for example, which of course can occur while lying down, intracranial pressure temporarily increases and produces high flow through the catheter. A weighted ball valve does not stop flow while the patient is recumbent because the ball can roll away from the seat. Therefore, a conventional weighted ball, antisiphoning valve is not able to protect against CSF surges while the patient is in a recumbent attitude.
In a weighted ball antisiphoning valve the flow enters from the bottom to assure that the ball will sink toward the seat in the upright orientation. Bottom entry causes the fluid to follow an S-shaped path from the brain to the abdomen. That is, fluid must travel downward to descend below the bottom seat, then upward past the seat and ball, and downward again to its ultimate destination. The shunt valve disclosed in U.S. Pat. No. 5,042,974 exemplifies such S-shaped fluid flow path of a bottom entry, weighted ball, antisiphoning valve. To accommodate the S-shaped path, conventional antisiphoning valves are bulky. Due to its size, the valve tends to remain in fixed position when implanted in the patient. As the patient grows, the distance between the ends of the catheter and the valve increase which stresses the elastic tubing normally used in VP shunts. This stress increases the probability that the tubing will disconnect from the shunt components and require surgical intervention to correct.
It is an object of the present invention to provide a streamlined, antisiphoning valve in which CSF flows straight through in a non-meandering path. Such a valve can have a smaller cross section than a conventional valve of equal capacity. The small cross section promotes the ability of the valve to move in the axial direction of the catheter within the body of the patient to relieve stress caused by the patient's growth.
It is another object of this invention to provide an attitude responsive, antisiphoning valve which can reduce surging fluid drainage even when the patient is recumbent.